Products of nifV, M, A, and F of Klebsiella pneumoniae have been identified, and suggestive evidence exists for the assignment of products for nifS, U, and L. These assignments confirm previous descriptions of transcriptional polarity and suggest that existence of several more nif genes whose products are apparently not essential for nitrogen fixation under standard laboratory conditions of growth. Although the nifA product is essential for expression of other nif transcripts, it is not involved in the regulation of its own transcript, nifRLA. The stability of nif-encoded proteins under anaerobic conditions was examined in a variety of mutants, confirming previously reported protein-protein interactions and suggesting several new ones. The stability of nif-encoded proteins in ammonia was found to be comparable to that in ammonia-free conditions, whereas all nif-gene products appeared to be labile in the presence of oxygen.